Mexico's Sergio Perez took his Force India to top place in pre-season testing at Bahrain's Sakhir circuit on Thursday while world champions Red Bull's winter woes continued.
As the final four-day testing period got underway, Perez clocked a best time of 1min 35.290sec with Valtteri Bottas continuing Williams' early promise with the second-best time.
Kimi Raikkonen in a Ferrari, Mercedes' Nico Rosberg, Adrian Sutil in a Sauber and promising McLaren driver Kevin Magnussen filled out the top six.
"We really needed a good day to start this week and today we had it," said Perez after completing a marathon 105 laps.
"We managed to do a lot of laps, try several different compounds and mappings, and it's been a fantastic day in terms of learning about the car."
But with the season-opening Australian Grand Prix just two weeks away, Red Bull again endured a frustrating session.
Australian driver Daniel Ricciardo clocked the team's best time in Bahrain this winter but he was still down in seventh place.
He managed 32 laps in the morning but just seven more in the afternoon before being confined to the garage as the car endured more technical glitches.
"We started really well this morning and put in some very useful laps straight away, which was good. We were able to run consistently and without issue and the plan was to increase the duration of the runs incrementally," said Red Bull engineering co-ordinator, Andy Damerum.
"That was all going according to plan. We sent Daniel out for a long-ish run, but we had to abort about five laps in, as we noticed some issues on the sensors.
"We brought him in, had a look at the car and discovered that we had a problem with the exhaust. So we had a long afternoon in the garage, but we did manage to get Daniel back out at the end of the session, which was good."
Ricciardo tried to remain positive with his final day in the seat set for Friday before world champion Sebastian Vettel completes driving duties on Saturday and Sunday.
"We still had a few issues today, but at least when the car was on track it was much more useful running and we made progress in the morning. It's getting there," said Ricciardo.